1. Field of the Invention
The invention relates to airspace supervision radar systems comprising a primary radar and a secondary radar; it relates more precisely to an electronic device for quantifying, at the output of the receiver of the secondary radar, pulse signals which form the responses transmitted by the aircraft under supervision.
2. Description of the Prior Art
In an airspace supervision radar system, the secondary radar, on the one hand, interrogates the aircraft in flight equipped with a cooperative responder and, on the other hand, picks up in return the responses transmitted by these aircraft. The bases of secondary radar and the international Conventions relating thereto are given, more especially, in the work by M. I. SKOLNIK "Radar Handbook", Chapter 38, (McGRAW-HILL 1971).
The response signal, or more briefly the response generated by the secondary radar responders, is a train of identical pulse comprising a number N of pulses which form a digital code; these code pulses are bracketed by two framing pulses which allow the response to be recognized and the beginning and the end of the code word to be identified. The responses transmitted by the different aircraft are specific to each plane and provide complementary and/or redundant information to that given by the echoes of the primary radar. These responses are picked up, on the ground, by the antenna of the secondary radar, then amplified and demodulated by the receiver. Prior to the operations for using these responses, the output pulse signals of the receiver must be reshaped, that is separated from the parasite signals then level-quantified.
The operation for quantifying the output signals of the receiver raises several difficulties, resulting more particularly from: the asynchronous responses from responders interrogated by adjacent secondary radars; the broad amplitude dynamics of the responses related to the gain parameters of the air-ground connection; at least partial superimposition of the responses coming from aircraft grouped together; parasite electromagnetic interferences and the noise of the receiver itself.
The problem which is raised is to construct a quantification device in which the quantification threshold is controlled by the instantaneous amplitude of the signals and in which the position of the sides of the output signals is, in the presence of multiple signals, scarcely erroneous. More briefly, the function of the quantification device is to restore as accurately as possible the shape of the pulses of each of the responses transmitted by the aircraft interrogated.
Quantification devices have already been constructed, they comprise essentially: a level comparator comprising a first input, or signal input, connected to the video frequency output of the receiver and a second input, or reference input, connected to a fixed or variable DC voltage source and generally controlled by the level of the thermal noise of the receiver. These quantification devices of the prior art present several disadvantages: the duration of the quantified signals depends on the absolute amplitude of the input signals, the result is that the position of the sides of the pulses is erroneous. Satisfactory adjustment of the reference level can never be achieved because too high a level leads to a loss of sensitivity at the range limit and conversely, too low a level introduces a widening of the pulses of the responses in the case of a high-amplitude response.